U.S. patent number 6,728,173 [Application Number 10/131,225] was granted by the patent office on 2004-04-27 for information reproducing method which reproduces plural components of a focus error signal and a tracking error signal from plural detection regions of plural reflected light beams.
This patent grant is currently assigned to Hitachi, Ltd.. Invention is credited to Yukio Fukui, Masayuki Inoue, Shigeru Nakamura, Kunikazu Ohnishi, Takeshi Shimano.
United States Patent |
6,728,173 |
Shimano , et al. |
April 27, 2004 |
Information reproducing method which reproduces plural components
of a focus error signal and a tracking error signal from plural
detection regions of plural reflected light beams
Abstract
An information reproducing method for reproducing information
recorded on a medium by irradiating a laser beam and detecting a
reflected light beam from the medium. The method includes detecting
a plurality of reflected light beams in which the polarities of
variations in reflected light beam intensity distributions are
substantially inverted to each other, the distribution variation
being produced when a light spot of the laser beam crosses a track
on the medium, generating components of a focus error signal and
components of a tracking error signal, adding the components of the
focus error signal of the plurality of reflected light beams to
generate a focus error signal, and taking a difference between the
components of the tracking error signal of the plurality of
reflected light beams to generate a tracking error signal.
Inventors: |
Shimano; Takeshi (Tokorozawa,
JP), Nakamura; Shigeru (Tachikawa, JP),
Ohnishi; Kunikazu (Yokosuka, JP), Inoue; Masayuki
(Yokohama, JP), Fukui; Yukio (Machida,
JP) |
Assignee: |
Hitachi, Ltd. (Tokyo,
JP)
|
Family
ID: |
12362277 |
Appl.
No.: |
10/131,225 |
Filed: |
April 25, 2002 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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249290 |
Feb 12, 1999 |
6400664 |
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Foreign Application Priority Data
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Feb 16, 1998 [JP] |
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10-032559 |
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Current U.S.
Class: |
369/44.37;
G9B/7.113; G9B/7.117; G9B/7.134; G9B/7.092; G9B/7.091; G9B/7.089;
G9B/7.031; 369/44.35; 369/44.41 |
Current CPC
Class: |
G11B
7/094 (20130101); G11B 7/0943 (20130101); G11B
7/1353 (20130101); G11B 7/1381 (20130101); G11B
7/131 (20130101); G11B 7/00718 (20130101); G11B
7/0941 (20130101); G11B 7/1367 (20130101); G11B
7/0903 (20130101); G11B 2007/0006 (20130101); G11B
7/0909 (20130101) |
Current International
Class: |
G11B
7/007 (20060101); G11B 7/135 (20060101); G11B
7/09 (20060101); G11B 7/13 (20060101); G11B
7/00 (20060101); G11B 007/00 () |
Field of
Search: |
;369/44.25-44.27,44.29,44.35-44.37,44.41 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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6-68496 |
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Mar 1994 |
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JP |
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07-225963 |
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Aug 1995 |
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JP |
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8-279166 |
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Oct 1996 |
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JP |
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Other References
Abstract JP 09.016972 dated by applicants..
|
Primary Examiner: Psitos; Aristotelis
Attorney, Agent or Firm: Antonelli, Terry, Stout &
Kraus, LLP
Parent Case Text
CROSS REFERENCE TO RELATED APPLICATION
This is a continuation of U.S. application Ser. No. 09/249,290,
filed Feb. 12, 1999, now U.S. Pat. No. 6,400,664, the subject
matter of which is incorporated by reference herein.
Claims
What is claimed is:
1. An information reproducing method for reproducing information
recorded on a medium by irradiating a laser beam and detecting
reflected light beams from the medium, comprising the steps of:
detecting with a plurality of detection regions a plurality of
reflected light beams in which polarities of intensity distribution
variations thereof are substantially inverted to each other, the
distribution variation being produced when a light spot of said
laser beam crosses a track on said medium; generating for each of
the detection regions plural components of a focus error signal for
a respective one of the plurality of reflected light beams and
plural components of a tracking error signal for the respective one
of the plurality of reflected light beams; adding the components of
the focus error signal of the detection regions to generate a focus
error signal; and taking a difference between the components of the
tracking error signal of the detection regions to generate a
tracking error signal; wherein said tracking error signal is
obtained by amplifying each of the components of the tracking error
signal of said plurality of reflected light beams with a gain
proportional to a ratio of reciprocal of a total amount of each
reflected light beam and then obtaining a difference between the
amplified components of the tracking error signal.
2. An information reproducing method according to claim 1, wherein
said medium has a groove width equal to approximately a half of a
track pitch.
3. An information reproducing method according to claim 1, wherein
the medium includes a plurality of grooves which are separated from
one another, and a light reflection when said light spot is in one
of said grooves is substantially equal to that when said light spot
is between grooves.
Description
BACKGROUND OF THE INVENTION
The present invention belongs to an optical head used in an optical
disk device, and more particularly relates to a technique for
enhancing a performance in detection of a position controlling
signal for an optical spot thereof.
Conventional techniques on methods for controlling a focal point
position in an optical disk device are described in, for example,
"Fundamentals and Applications of Optical Disk Storage", Y.
Tsunoda, 1995, 1st edition (Korona Corp., Tokyo), pp. 79-83.
According to this literature, there are the following methods:
Foucault method (Knife edge method), an astigmatic method, a beam
size detection method, an image rotating method, and so on. From
criteria such as simplicity of an optical system required, the ease
with which the adjustment can be made, and the ease with which
combination with a tracking detection can be achieved, the most
prevailing method, at the present stage, is the astigmatic method.
In the astigmatic method, however, there existed a problem that,
when an optical spot crosses a track on the surface of a storage
film, a disturbance is apt to occur in a focus error signal in
association with a decentering of an optical disk. This disturbance
is likely to occur especially when astigmatism takes place in a
focused spot or the optical spot is shifted on an optical detector.
Examples of methods for reducing the disturbance are disclosed as
follows: A method of reducing the disturbance by blocking light out
of a central portion of a detected light beam is disclosed in
JP-A-6-162527 and JP-A-6-309687, a method of reducing it by
adjusting rotation of an objective lens is disclosed in
JP-B-5-68774, and a method of reducing it by means of an operation
between a light with astigmatism and a light without astigmatism in
a detected system is disclosed in JP-A-5-197980. None of them,
however, is a fundamental method for solving the above-described
problem. Thus, at the present stage, the reducing effect obtained
is not necessarily enough.
In particular, in a land-groove type optical disk employed in a
DVD-RAM planned to be brought into a commercial stage soon, the
disturbance occurs quite outstandingly. The reason is as follows:
In the land-groove type optical disk, a width of a guiding groove
(groove) is substantially equal to a width of a portion of a
guiding inter-groove (land), and information is stored on the both
sides thereof. On account of this, a pitch of the guiding groove
itself, when compared with an optical spot, is formed to be larger
than in conventional optical disks. This extraordinarily
intensifies a tracking error signal according to a push-pull method
described later, thus causing the disturbance to occur quite
outstandingly. This condition, accordingly, brings about a
situation that, in an optical head for the DVD-RAM, it can not be
helped employing the Foucault method or the beam size detection
method the configuration or the adjustment of which is
complicated.
Conventional techniques on methods for controlling a tracking in an
optical disk device are similarly described in, for example, the
above-cited "Fundamentals and Applications of Optical Disk
Storage", Y. Tsunoda, 1995, 1st edition (Korona Corp., Tokyo), pp.
83-92. According to this literature, there are methods such as a
three-spots method and a diffracted light differential method
(push-pull method). Judging from criteria such as simplicity of an
optical system required, the ease with which the adjustment can be
made, and a resistance to the disturbance, the three-spots method
is mainly employed in a read only type optical disk such as a
compact disk (CD). Meanwhile, the push-pull method is mainly
employed in the case of a magneto-optical disk or the DVD-RAM which
needs a high laser emission power at the time of the recording. At
this time, there can be considered another way in which, exchanging
the roles with each other, the push-pull method is employed toward
the CD and the three-spots method is employed toward writable
optical disks. However, there exist circumstances which make such
an employment impossible.
In performing a CD pick up, in order to cause a focused spot to
follow a decentering of the optical disk for the necessity of low
price, the objective lens is moved by only being mounted on a lens
actuator. Then, if the push-pull method is employed, it turns out
that a detected light beam moves on the optical detector. This
phenomenon appears as an off-set. Also, at a pit depth of
.lambda./(4n) (.lambda.: light wavelength, n: substrate refractive
index) at which a signal amplitude becomes largest in the
reproduction-only type optical disk, there are the following
problems: Of diffracted light by means of a periodic structure of
train of pits in the radial direction, 0th order light becomes
smaller. In addition to this, even when the focused spot is
off-track, no unbalance occurs in interference intensity between
the 0th order light and .+-.1st order diffracted lights. This makes
it impossible to obtain the tracking error signal.
Meanwhile, in the storage-able optical disks, especially in the
magneto-optical disk, compensation for the decentering of the
optical disk is usually performed by an actuator called a coarse
actuator. The coarse actuator mounts the optical head or only a
portion of the objective lens and an objective lens actuator so as
to allow the optical spot to come near to a proximity of a track to
be objected. Namely, the magneto-optical disk is constituted in
such a manner that, of a tracking error, the low frequency
components are compensated by the coarse actuator and the high
frequency components are compensated by the objective lens
actuator, thereby enhancing a reliability needed for the storage
operation. Accordingly, an amount of movement by the objective lens
actuator is lower than in the read only type optical disk such as
the CD. This makes it possible to employ the push-pull method which
has higher light utilization efficiency than the three-spots method
does.
Also, if the three-spots method is employed toward the storage-able
optical disks, as described on page 127 of "Technical Digest of
Symposium on Optical Memory '86", there take place the following
problems: First, in an optical disk such as the DVD-RAM, i.e. the
type of optical disk that performs the storage by means of a
variation in reflectance of a storage mark, at the time of the
storage operation, there arises a difference in the amount of light
between a preceding sub-spot and a subsequent sub-spot. This causes
an off-set to occur in the tracking error signal. Also, in the case
of the magneto-optical disk, there exists a feedback light back to
a semiconductor laser. On account of this, a tilt of the disk
unbalances a condition of stray-lights interference on the both
sides of sub-spots. This also causes an off-set to occur. Moreover,
as described already, the land-groove type optical disk is employed
in the DVD-RAM. This circumstance can also be mentioned as a reason
for making it impossible to employ the three-spots method toward
the DVD-RAM. Namely, in the land-groove type optical disk, a width
of the land portion is originally made equal to that of the groove
portion in order to make an amount of reflected light of the land
portion equal to that of the groove portion. This necessarily
results in a fact that, even when an optical spot is off-track, the
amount of light scarcely varies, thus making it impossible to
obtain a tracking error signal according to the three-spots method.
Accordingly, it can not be helped employing the push-pull method in
the DVD-RAM. However, unlike the case of the magneto-optical disk,
it is required to lower the price of the DVD-RAM down to a price
close to the price of the CD. Consequently, it becomes absolutely
necessary to reduce the off-set in a tracking error signal which
accompanies the movement of the objective lens according to the
push-pull method.
A conventional technique for solving the above-mentioned problem in
the DVD-RAM is described in, for example, "National Technical
Report", Vol. 40, No. 6, (1994), pp. 771-778. Here, the optical
disk device is constituted as follows: The objective lens, a
.lambda./4 plate, and a polarizing diffraction grating are
integrally mounted on an objective lens actuator. Moreover, the
polarizing diffraction grating is constituted so that interference
regions, in which, of diffracted light by mean of the disk, -1st
order diffracted light and -1st order diffracted light each
interfere with 0th order light, are diffracted with a different
diffraction angle, respectively. This constitution makes it
possible to separate, on the optical detector, the interference
region between the +1st order diffracted light and the 0th order
light from the interference region between the -1st order
diffracted light and the 0th order light. From this, the
above-mentioned literature shows the following: If a dual-divided
optical detector is constituted so that, when the objective lens is
moved, the lights do not stray out of the optical detector, it
becomes possible to eliminate the off-set caused by the phenomenon
that the optical spots move on the optical detector.
Also, employing the polarizing diffraction grating as a diffraction
grating makes the following possible: When a light heading for the
disk passes through the polarizing diffraction grating, the
diffraction efficiency is made substantially equal to zero, and
when a reflected light from the disk passes through the polarizing
diffraction grating again, the diffraction efficiency is caused to
become an appropriate value. Meanwhile, in the case of a
non-polarizing ordinary diffraction grating, it diffracts the light
heading for the disk, too, thus making it impossible to avoid a
loss of the amount of light. Employing the polarizing diffraction
grating in this way allows only the necessary diffraction of the
reflected light to occur, thus making it possible to prevent the
loss of the amount of light.
However, in this conventional example, the objective lens, the
.lambda./4 plate, and the polarizing diffraction grating are
integrally mounted on the objective lens actuator. This
constitution results in a problem that a movable portion of the
actuator becomes heavy, thus restricting a response speed of the
actuator down to a low level. Since optical disks are being
improved in the storage density and at the same time are becoming
faster in the transfer rate year by year, the above-described
conventional example is not able to meet the trend of even further
speeding-up in the near future.
Another method, which, with no other optical component except the
objective lens mounted on the objective lens actuator, makes it
possible to eliminate the tracking error signal off-set which
accompanies the movement of the objective lens according to the
push-pull method, is disclosed in the above-described "Technical
Digest of Symposium on Optical Memory '86", pp. 127-132. This
method is referred to as a differential push-pull method. In the
method, the three-spots method is employed, and respective tracking
error signals according to the push-pull method are subtracted on a
main-spot and two sub-spots, thereby eliminating the tracking error
signal off-set which accompanies the movement of the objective
lens. Namely, in the method, the sub-spots are located in such a
manner that they are shifted on the both sides of the main spot by
one-half of a period of the guiding groove, thereby simultaneously
detecting a light beam in which variations in interference
intensity distribution of a reflected light beam reflected from the
disk in association with an off-track are inverted, and thus
generating opposite phase tracking error signals which contain the
off-set in the same phase. Then, these opposite phase tracking
error signals are subtracted, thereby allowing only the off-set to
be cancelled. According to this conventional example, the ratio of
the gain to amplify the signal by the main spot to the gain of the
signal by the sub-spot is chosen so as to compensate the intensity
unbalance caused by diffraction efficiency characteristics of the
diffraction grating to generate sub-spots. The use of this
conventional example, with no other optical component except the
objective lens mounted on the objective lens actuator, basically
makes it possible to eliminate the tracking error signal off-set
which accompanies the movement of the objective lens according to
the push-pull method. In the present conventional example, however,
no countermeasure is taken against the mixture of the disturbance
into the focus error signal when a focused spot crosses the guiding
groove in the astigmatic focus error detecting method described
earlier. Also, as described in the present conventional example,
when one of the sub-spots lies in a post-stored track and the other
lies in a pre-stored track, the effect of reducing the off-set is
not enough. Further, although not described in the present
conventional example, when a total amount of reflected light on the
guiding grooves differs from a total amount of reflected light on
the guiding inter-grooves, the off-set also remains in the present
conventional example. This situation arises when a width-of the
guiding groove is not equal to that of the guiding inter-groove.
However, in the case of the DVD-RAM employing the land-groove type
optical disk in which the width of the guiding groove is equal to
that of the guiding inter-groove, such a situation also arises if
the main-spot lies in the post-stored track and the two sub-spots
lies in the pre-stored track or in the case opposite thereto. Still
further, in the present conventional example, there exist the
plurality of optical spots. This brings about a disadvantage in the
light utilization efficiency at the time of the storage.
Moreover, the gains to amplify the signals by main spot and
sub-spots chosen in this conventional method is insufficient to
cancel the effect completely. The reason is as follows. As
described later, when a width of the guiding groove does not
substantially equal to half of the pitch of guiding grooves, the
reflectance of the light when the focused spot is at the guiding
groove is different from that when the focused spot is at the
inter-groove. It is also necessary to compensate this unbalance of
reflectance for perfect offset cancellation. For the higher the
density of the optical disk, the allowance of the offset is the
severer. Therefore this insufficient cancellation must be a
problem, recently.
Still more, in this conventional example, the optical disk was not
a land-groove type optical disk. Therefore, there is no cross-talk
from stored information signal, because the sub-spots on the
optical disk are not on the information track at readout process.
In the case of land-groove type optical disk such as DVD-RAM,
however, the sub-spots is also on the information track of reading
out from the disk. This results in disturbance to the tracking
error signal.
Another method, which cancels the disturbance in the focus error
signal of astigmatic method, is disclosed in the JP-A-4-168631.
Also in this method, the main spot and sub-spots by a diffraction
grating is positioned onto the optical disk at the distances of the
half of the pitch of guiding grooves in the radial direction of the
disk. The reflected beams from these focused spots pass through a
cylindrical lens, then detected by three quadratic photo-detectors,
respectively. From the output signal of these photo detectors,
three focus error signals are obtained by calculation in the
electric circuit. These focus error signals are amplified with
gains which are proportional to the reciprocals of the light
intensity of each focused spot on the optical disk, which are not
proportional to the reciprocal of the reflected light intensity.
Then summation of these amplified focus error signals is calculated
in the electric circuit. Employing this method, the extra
disturbance to the conventional focus error signal by aberrations
or miss-alignment of the optical components or photo detector can
be eliminated. The optimum gains for disturbance cancellation for
this method is different from those for differential push-pull
method as mentioned. However, in this method, no tracking method is
disclosed. Further more, if the differential push-pull method
described in the conventional literature itself is employed with
this focusing error detection method, it is necessary to set the
gains to amplify the signal by each reflected light beam equal
between in the focus error signal and in the tracking error signal,
namely proportional to the reciprocals of the light intensity of
incident focused spots on to the optical disk. It results in the
insufficient cancellation of the offset of tracking error signal as
mentioned.
SUMMARY OF THE INVENTION
In view of the above-described conventional techniques, in the
method and the device for detecting the focal point shift, a
problem to be solved by the present invention is to fundamentally
eliminate the disturbance which occurs in the focus error signal in
association with the decentering of an optical disk when an optical
spot crosses a track on the surface of the storage film.
Also, another problem to be solved by the present invention is to
fundamentally cancel the off-set which occurs simultaneously in the
tracking error signal in association with the movement of the
lens.
Also, when employing a method such as the differential push-pull
method in which a light beam, in which variations in interference
intensity distribution of a reflected light beam at the time when
an optical spot on the disk crosses the guiding groove are
inverted, is generated simultaneously with the ordinary light beam
and thus the opposite phase tracking error signals which contain
off-set components with the same phase are generated so as to cause
the same phase off-set to be cancelled, another problem to be
solved by the present invention is to cancel an off-set which
occurs from a difference in the total amount of reflected light
between these light beams.
Also, another problem to be solved by the present invention is not
only to cancel, in the differential push-pull method, the off-set
which occurs in the tracking error signal in association with the
movement of the lens but also to fundamentally eliminate, in the
focal point shift detecting method, the disturbance which occurs in
the focus error signal in association with the decentering of an
optical disk when an optical spot crosses a track on the surface of
the storage film.
Also, another problem to be solved by the present invention is to
obtain, with the sub-spots in the differential push-pull method
located on the same track as the main-spot, the same effect of
canceling the tracking error signal off-set which accompanies the
movement of the objective lens.
Also, another problem to be solved by the present invention is to
constitute the optical disk device so that a single spot on the
disk exhibits the same effect as the differential push-pull method
does.
Also, another problem to be solved by the present invention is to
obtain these effects toward the astigmatic focal point shift
detecting method and the push-pull tracking detecting method in
particular.
Also, another problem to be solved by the present invention is to
illustrate a configuration of an optical detector which allows
these effects to be obtained.
Also, another problem to be solved by the present invention is to
enhance performance in the canceling of the tracking error signal
off-set due to the movement of the objective lens when combining
the differential push-pull method with the additive astigmatic
method.
Also, another problem to be solved by the present invention is to
eliminate an influence of the disturbance due to the information
pits when combining the differential push-pull method with the
additive astigmatic method so as to apply them together to the
land-groove type optical disk.
In order to solve the above-described problems, an optical head
comprises at least a semiconductor laser, a light-converging
optical system for converging an emitted light from the
semiconductor laser onto an optical disk having a periodic
structure in a radial direction as at least one focused spot, an
optical detection system for detecting a reflected light from the
optical disk, and an electrical circuit for calculating an amount
of received light through a photoelectric conversion thereof so as
to obtain at least one of a focus error signal of the focused spot
converged on the optical disk, a tracking error signal of the
focused spot converged on the optical disk, and a data signal
stored in the optical disk. The light-converging optical system
includes means for generating a plurality of reflected light beams
in which polarities of their intensity distribution variations at
the time when the periodic structure crosses the focused spot on
the optical disk are substantially inverted with each other, the
optical detection system includes means for splitting and
simultaneously detecting the plurality of reflected lights, and the
electrical circuit obtains the focus error signal by taking
summation of focus error signals of the respective reflected light
beams so that variations of the focus error signal caused by their
intensity distribution variations cancel out with each other.
Also, at this time, a difference between respective tracking error
signals of the plurality of reflected lights the polarities of
which are inverted with each other is simultaneously defined as the
tracking error signal.
Moreover, at this time, in defining, as the tracking error signal,
the difference between the respective tracking error signals of the
plurality of reflected light beams the polarities of which are
inverted with each other, in the electrical circuit, the respective
tracking error signals are amplified with a gain which is
proportional to a ratio between reciprocals of respective total
amounts of the reflected lights when one of said focused spot is at
the information track of said optical disk, and after that a
difference between the respective tracking error signals thus
amplified is calculated, then being defined as the tracking error
signal.
Also, in these constitutions, there is provided a beam splitting
device for splitting the reflected light beam from the optical disk
off from an optical path extending from the semiconductor laser,
and the means for generating said plurality of reflected light
beams the polarities of intensity distribution variations of which
are substantially inverted with each other is a diffraction grating
located between the semiconductor laser and the beam splitting
device. Moreover, the diffraction grating is installed in such a
manner as to be tilted toward the radial direction of the optical
disk so that focused spots of .+-.1st order diffracted lights by
means of the diffraction grating are located in such a manner that,
on the optical disk and with reference to a focused spot of a 0th
order light, they are shifted by about one-half of a period of the
above-described periodic structure in opposite directions to each
other in the radial direction.
Also, there is provided a beam splitting device for splitting the
reflected light beam from the optical disk off from an optical path
extending from the semiconductor laser, and the means for
generating the plurality of reflected light beams the polarities of
intensity distribution variations of which are substantially
inverted with each other is a diffraction grating located between
the semiconductor laser and the beam splitting device. Moreover,
the diffraction grating has gratings the phase of which is inverted
at an interval of substantially .lambda.D/(2NA.multidot.P)
(.lambda.: light wavelength, NA: numerical aperture of an objective
lens, P: period of the periodic structure in the radial direction
on the optical disk, D: effective light beam diameter on the
diffraction grating) in regions of a common width in the radial
direction on the optical disk. The diffraction grating is installed
in such a manner that a direction of the gratings thereof is in
parallel to a tangential direction of the optical disk so that, on
the optical disk, focused spots of .+-.1st order diffracted lights
by means of the diffraction grating are located on the same track
as a focused spot of a 0th order light. Furthermore, the optical
detection system splits and detects these focused spots. Then, a
data signal is obtained from an amount of received light signal
resulting from the 0th order light.
Still further, there is provided a beam splitting device for
splitting the reflected light beam from the optical disk off from
an optical path extending from the semiconductor laser, and the
means for generating the plurality of reflected light beams the
polarities of intensity distribution variations of which are
substantially inverted with each other is a polarizing phase
shifter located between the semiconductor laser and the beam
splitting device. The polarizing phase shifter is constituted so
that it relatively inverts a phase of a linearly polarized light
component, which is polarized in a specific direction, at an
interval of substantially .lambda.D/(2NA.multidot.P) (.lambda.:
light wavelength, NA: numerical aperture of an objective lens, P:
period of the periodic structure on the optical disk, D: effective
light beam diameter on a diffraction grating) in regions of a
common width in the radial direction on the optical disk, and a
phase of a linearly polarized light component perpendicular to the
linearly polarized light component is not varied over a whole
system of the polarizing phase shifter. Furthermore, the optical
detection system splits and detects these polarized light
components with the use of a polarizing beam splitting device.
Then, a data signal is obtained from the polarized light component
to which no phase inversion is added.
In particular, the above-described constitutions are embodied by
employing the astigmatic method for the focus error detection and
by employing the push-pull method for the tracking error
detection.
Also, in the optical detection system, there is provided an optical
detector in which there exist at least two sets of optical
detection regions each of which receives a single optical spot with
a four-divided optical detection region.
Also, an optical head includes a semiconductor laser, a
light-focusing optical system for focusing, as at least one focused
spot, an emitted light from the semiconductor laser onto an optical
disk which has a periodic structure such as guiding grooves in its
radial direction, an optical detection system for detecting a
reflected light from the optical disk, and an electrical circuit
for obtaining, from the reflected light, both a focus error signal
of one of the focused spots and a tracking error signal thereof. In
the optical head, sub-spots, for example, are located by an
apparatus such as a diffraction grating in such a manner that they
are shifted from a main-spot by one-half of a period of the guiding
grooves, thereby generating two kinds of and, for each of the
kinds, at least one or more of reflected light beams in which
polarities of their intensity distribution variations at the time
when the periodic structure crosses the focused spots are
substantially inverted with each other. The optical detection
system splits and detects the plurality of reflected light beams.
In the electrical circuit, focus error signals, which are obtained
by each adding focus error signals generated from the two kinds of
and, for each of the kinds, at least one of reflected light beams,
are amplified and added further, thereby obtaining the focus error
signal. Moreover, tracking error signals, which are obtained by
each adding tracking error signals generated from the two kinds of
and, for each of the kinds, at least one of reflected light beams,
are amplified and subtracted from each other, thereby obtaining the
tracking error signal. At this time, an optical disk such as the
land-groove type optical disk is used in which the guiding grooves
constitute the periodic structure and, as compared with an occasion
when one of the focused spots is situated at a guiding groove, an
error of the reflectance on an occasion when it is situated at a
guiding inter-groove falls within a range of .+-.10% thereof. The
use of such type of optical disk makes it possible to cause an
amplification gain ratio between the tracking error signals of the
two kinds of reflected light beams to coincide with an
amplification gain ratio between the focus error signals of the two
kinds of reflected light beams.
Also, in a similar optical head, in a case where the optical disk
used is an optical disk other than the land-groove type optical
disk, i.e., in a case where the reflectances differ between an
occasion when one of the focused spots is positioned on an
information track of the optical disk and an occasion when it is
positioned at a position which is apart from the information track
by one-half of the period of the periodic structure, it turns out
that the amplification gain ratio between the tracking error
signals of the plurality of reflected light beams differs from the
amplification gain ratio between the focus error signals of the
plurality of reflected light beams.
Also, in the optical head, the electrical circuit for detecting the
sub-spots is provided with a frequency characteristic which makes
it possible to cut off a frequency bandwidth of a read-out signal
of recorded information written in the optical disk.
Other objects, features and advantages of the present invention
will become apparent from the following detailed description of the
embodiments of the invention taken in conjunction with the
accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a diagram for showing a constitution of an optical system
in a basic embodiment of the present invention;
FIG. 2 is a diagram for showing locations of focused spots on an
optical disk and intensity distributions of reflected light beams
at that time;
FIG. 3 is a diagram for showing a circuit calculation method of an
output of a detector;
FIG. 4 is a diagram for showing a calculation method of the output
of the detector;
FIG. 5 is a diagram for explaining a tracking error signal off-set
due to movement of an objective lens;
FIG. 6 is a diagram for explaining an disturbance into a focus
error signal due to astigmatism;
FIG. 7 is a diagram for showing a constitution of an optical system
in an embodiment in which a phase-inverted diffraction grating is
employed;
FIG. 8 is a diagram for explaining a detailed structure of the
phase-inverted diffraction grating;
FIG. 9 is a diagram for explaining a manner in which phase shift
regions are overlapped in diffraction of a phase-inverted light by
means of optical disk guiding grooves;
FIG. 10 is a diagram for showing interference phase differences
added to an optical disk diffracted light by means of the
phase-inverted diffraction grating;
FIG. 11 is a diagram for explaining a manner in which, when the
movement of the objective lens exists, phase shift regions are
overlapped in diffraction of a phase-inverted light by means of
optical disk guiding grooves;
FIG. 12 is a diagram for showing a constitution of an optical
system in an embodiment of the present invention in which a
polarizing phase shifter is employed;
FIG. 13 is a diagram for explaining the principle of the polarizing
phase shifter;
FIG. 14 is a diagram for explaining calculation of an disturbance
due to an ordinary crossing over a guiding groove by means of a
focus error signal;
FIG. 15 is a diagram for explaining calculation of an disturbance
due to a crossing over a guiding groove by means of a focus error
signal at the time of employing the phase-inverted diffraction
grating;
FIG. 16 is a diagram for explaining calculation of an disturbance
due to a crossing over a guiding groove by means of a focus error
signal in a differential push-pull method;
FIG. 17 is a diagram for explaining lens shift characteristics of a
tracking error signal with the use of an ordinary detecting light
beam for a focus error signal;
FIG. 18 is a diagram for explaining lens shift characteristics of a
tracking error signal with the use of a detecting light beam for a
focus error signal at the time of employing the phase-inverted
diffraction grating;
FIG. 19 is a diagram for explaining lens shift characteristics of a
tracking error signal with the use of a detecting light beam for a
focus error signal in the differential push-pull method;
FIG. 20 is a diagram for showing an embodiment of an optical system
constitution according to the present invention in which the
reproduction is possible in a DVD, a DVD-RAM, a CD, and a CD-R;
FIG. 21 is a diagram for explaining details of a constitution for
the detection in the embodiment in FIG. 20;
FIG. 22 is a diagram showing a modified example of the circuit
calculation method of the detector output in FIG. 3; and
FIG. 23 is a characteristic diagram representing a frequency
characteristic of a gain of an amplifier and a read-out signal
intensity of the detector.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
The description will be given below concerning embodiments of the
present invention, using the accompanying drawings.
FIG. 1 is a diagram for showing a constitution of an optical system
in a basic embodiment of the present invention. A light beam
emitted from a semiconductor laser 101 produces a diffracted light
by passing through a diffraction grating 102. The diffracted light,
by way of a beam splitter 103, a triangle reflection prism 104 and
an objective lens 106, forms a main-spot 108 of a 0th order light
and two sub-spots 109, 110 of .+-.1st order diffracted lights on an
optical disk 107. A reflected light beam, by way of the objective
lens 106 and the triangle reflection prism 104 again, is reflected
at the beam splitter 103. Then, the reflected light beam is
provided with an astigmatism for detecting a focal point shift by a
cylindrical lens 111, thus being received by an optical detector
115. The optical detector 115 is separated into a 0th order light
four-divided optical detection region 112 and .+-.1st order
diffracted lights four-divided optical detection regions 113, 114.
The two kinds of optical detection regions are independent of each
other in performing the detection. Here, the diffraction grating
102 is located in such a manner as to be tilted to some extent so
that the .+-.1st order diffracted lights on the optical disk are
located in such a manner as to be shifted on the both sides of the
0th order light by one-half of a guiding groove pitch.
FIG. 2 is a diagram for showing locations of optical spots on the
optical disk and intensity distributions of reflected light beams
at that time. FIG. 2 shows a case in which a 0th order light 201
and .+-.1st order diffracted lights 202, 203 are shifted slightly
on the left side with reference to groove portions 204 and land
portions 205, a case in which they are just on-track, and a case in
which they are shifted slightly on the right side. At this time,
intensity variations of a 0th order light-detected light beam 206
and a .+-.1st order diffracted lights-detected light beam 207, as
illustrated in FIG. 2, are shifted in directions opposite to the
directions of the above-mentioned track-shifts of the optical spots
on the disk. This is because the .+-.1st order diffracted lights
202, 203 on the disk are located in such a manner that they are
sifted with reference to the 0th order light 201 by one-half of the
track pitch. There occur such light intensity distributions of the
detected light beams in correspondence with positions of the
optical spots on the optical disk. As described in, for example,
the literatures cited earlier, this knowledge itself has been known
publicly.
FIG. 3 is a diagram for showing a circuit calculation method of
outputs of the optical detector. Incidentally, on the optical
detector 115, the intensity distributions of the reflected light
beams are rotated by 90 degrees because of the astigmatism for
detecting a focal point shift or focus error. Here, a focus error
signal (AF signal) is obtained by adding components in the same
diagonal direction of corresponding divided outputs of the divided
optical detector 112 for the main-spot and the divided optical
detectors 113, 114 for the sub-spots, and then by calculating the
differential signal thereof with the use of a differential
amplifier 303. Calculating the focus error signal in this way
allows only the disturbance to be canceled out because, when the
focused spots cross a guiding groove on the disk, variations in
intensity distribution of the sub-spots are inverted with a
variation in intensity distribution of the main-spot. At this time,
usually, an amount of light of the sub-spots is made smaller than
that of the main-spot. Accordingly, the calculation is performed
after signal outputs of the sub-spots are amplified by an amplifier
301 by an amount corresponding to the ratio between the amount of
light of the sub-spots and that of the main-spot. In this
embodiment, however, there exist the two sub-spots. Assuming an
intensity of the main-spot as A and that of the sub-spot as B, the
gain on each sub-spot, actually may take a value obtained by
multiplying, by A/(2B), amplification gains of the signals by the
two sub-spots with reference to the main-spot. Meanwhile, a
tracking error signal (TR signal) is obtained by first adding,
alternately between in the main-spot and in the sub-spots, an
output for every two regions divided into the right and left in
FIG., 3, and then by calculating the differential output thereof
with the use of a differential amplifier 304. Calculating the
tracking error signal in this way makes it possible to obtain a
tracking error signal in which only the off-set components are
canceled out, because, when the focused spots cross the guiding
groove on the disk, the variations in intensity distribution of the
sub-spots are inverted with the variation in intensity distribution
of the main-spot and in addition the off-set due to the lens shift
is not inverted. Here, from the above-described original location,
when the main-spot is situated on a land portion, the sub-spots are
situated on groove portions. This, when a width of the land portion
is different from that of the groove portion, results in a
difference in the amount of reflected light between the main-spot
and the sub-spots, thereby making the off-set canceling
insufficient. In such a case, signals of the sub-spots are
amplified by an amplifier 302 so that the difference in the amount
of reflected light therebetween is compensated. For example, when
the information tracks exist on the land portions, the again of the
amplifier 302 of the sub-spots, may take a/b where a represents a
reflectance of the land portion as and b represents that of the
groove portion. Also, in some cases, the output of the main-spot
may be lower. In such a case, the main-spot, on the contrary, is
amplified. Otherwise, the gain of the amplifier 302 is made equal
to 1 or less. The above-described calculation method makes it
possible to simultaneously obtain the tracking error signal without
the off-set due to the lens shift and the focus error signal
without the disturbance at the time of crossing the guiding groove.
In the mean time, concerning a reproducing signal, a total amount
of light of the main-spot is outputted using a differential
amplifier 305. Incidentally, here, the optical disk employed is
assumed to be an optical disk such as a reproduction-only type
optical disk or a phase-varied type optical disk which allows a
signal to be reproduced with the use of the amount of reflected
light. However, in the case of the magneto-optical disk as well,
there exists no other difference except a difference which results
from defining the data signal as a differential signal between two
signal outputs in which the polarized light components are split.
Consequently, it is possible to detect the focus error signal and
the tracking error signal with the use of the present
embodiment.
FIG. 4 is a diagram for summarizing the calculation method at this
time. As a conclusion, what should be done is just to perform the
calculations as illustrated in FIG. 4 toward four outputs a, b, c,
d of the 0th order light four-divided optical detection region 112
and respective four outputs e, f, g, h, i, j, k, 1 of the .+-.1st
order diffracted lights four-divided optical detection regions 113,
114. Incidentally, here, a reference note RF denotes a data signal,
AF a focal point shift error signal, and TR a tracking error
signal.
The above-described embodiment has generally assumed the case in
which the reflectances differ between an occasion when a focused
spot is situated at the guiding groove and an occasion when it is
situated at the guiding inter-groove. However, in the land-groove
type optical disk used in the DVD-RAM disk, the width of the
guiding groove is substantially equal to one-half of the track
pitch, and thus the reflectances substantially equal between the
occasion when a focused spot is at the groove portion and the one
when it is at the land portion. This, by omitting the amplifier 302
in FIG. 3, makes it possible to simplify the circuit configuration
as illustrated in FIG. 22. Incidentally, even in the land-groove
type optical disk, because of the fabricating error, a difference
in the reflectance in the land portion toward the groove portion
can be about, at the maximum, .+-.10%. Concerning the difference of
this extent, however, the computer simulation has demonstrated the
following: When an effective diameter of the objective lens is set
to be 4 mm, even if the lens shift is 0.4 mm, the track off-set
turns out to be about 0.01 .mu.m in a DVD-RAM disk the groove pitch
of which is 1.48 .mu.m. This means that, in the configuration in
FIG. 22 as well, the track off-set is allowable. Conversely,
provided that the allowable value of the track off-set is equal to
0.05 .mu.m, the difference in the reflectance in the land portion
toward the groove portion has been found to be about 1.6 times
under the same conditions. This means that, in an ordinary optical
disk other than the land-groove type optical disk, this value
becomes more than two times greater. Accordingly, the configuration
in FIG. 22, after all, can be applicable only to the land-groove
type optical disk.
Also, in the land-groove type optical disk, there exist the
information tracks at the guiding grooves as well as at the guiding
inter-grooves. Consequently, when the main-spot is situated on an
information track, the sub-spots, naturally, are situated on the
adjacent tracks. At this time, there occurs a mixture of recorded
information into the sub-spots, which has not been assumed except
for the case of the land-groove type optical disk. In order to
avoid this phenomenon, it is appropriate to let an amplifier 301 in
FIG. 22 have a frequency characteristic as illustrated in FIG. 23.
In FIG. 23, a horizontal axis in the coordinate indicates the
frequency, a vertical axis on the left side indicates the frequency
characteristic of a gain of the amplifier, and a vertical axis on
the right side indicates an intensity of a read-out signal on
information tracks in a detector. Although the read-out signal lies
in a higher frequency bandwidth as compared with control signals
such as the tracking error signal or the focus error signal, a
signal actually detected by the detector is a one resulting from
synthesizing these signals. Here, by letting the amplifier have a
characteristic that the gain becomes lower in the read-out signal
bandwidth, it is possible to obtain a control signal without the
disturbance.
The optical disk in the embodiment mentioned upper was not
specialized to land-groove type optical disk. In the case of
land-groove type optical disk such as DVD-RAM, the reflectance of
the light is substantially equal when between the focused spot on
the optical disk is in groove and in inter-groove. Therefore the
amplifier 302 in the FIG. 3 can be omitted as in FIG. 22. Of
course, even in the land-groove disk, these reflectances have some
error approximately 10%. However, it is examined by computer
simulation that this amount of deviation is allowable.
In FIG. 23, the frequency spectrum of the readout signal, servo
control signal, and frequency transfer characteristics of the
amplifiers in FIG. 22 is shown. The amplifier has the frequency
transfer characteristics to substantially cut off the frequency
band of stored information signal in the optical disk.
Next, an analytical explanation will be given below concerning the
reason why such a calculation method makes it possible to cancel
the tracking error signal off-set due to the movement of the
objective lens. According to "Journal of Optical Society of
America", 1979, Vol. 69, No.1, pp.4-24, distribution of a reflected
light beam by means of the periodic structure of the optical disk
is obtained as follows: In scalar diffraction approximation,
products of Rm, i.e. m-th order Fourier coefficients of reflectance
distribution of the optical disk, and a (x, y), i.e. complex
amplitude distribution of an incident light beam, are shifted by a
quantity of mNA/P.lambda. (NA: numerical aperture, P: period of
guiding grooves, .lambda.: wavelength), i.e. distribution shift due
to a m-th order diffraction, and, after being multiplied by exp (i2
.pi.mu.sub.o /P), i.e. phase component based on a spot position of
the main spot u.sub.o, are added, thus obtaining the distribution.
Namely, the following Equation (1) is obtained: ##EQU1##
Here, Rm, which corresponds to a complex amplitude of a m-th order
diffracted light at the time when a parallel light beam with an
amplitude 1 is launched into the optical disk at an angle
perpendicular thereto, is represented by Equation (2): ##EQU2##
and, in particular, in the case of rectangular grooves with a width
w and a groove depth d normalized by the wavelength, Rm is
represented by Equation (3): ##EQU3##
Incidentally, here, sinc X has the relation expressed by Equation
(4): ##EQU4##
Using these equations, and provided that the incident light beam
has no aberration and the amplitude is uniform within an objective
lens pupil surface, interference intensities between the 0th order
light and the .+-.1st order diffracted lights by means of the
periodic guiding grooves in the optical disk are represented by
Equation (5): ##EQU5##
Incidentally, here, .phi. has the relation expressed by Equation
(6):
.phi.=arg(R.sub..+-.1)-arg (R.sub.0) (6)
Using these equations, a tracking error signal TR according to a
push-pull method at the time when there exists no movement of the
lens is represented by Equation (7): ##EQU6##
Here, as shown in FIG. 5, assuming that an optical spot 502 on a
dual-divided optical detector 501 is moved by the movement of the
objective lens, from an increase or a decrease in the amount of
received light in each divided region of the dual-divided optical
detector 501, a tracking error signal TR according to the ordinary
push-pull method is represented by Equation (8), using parameters
.alpha. (0<.alpha.<1), .beta.(0<.beta.<1) attributed to
the movement of the objective lens: ##EQU7##
The second term in the right hand side corresponds to the off-set.
Here, in Equation (5), if, for example, the sub-spots by means of
the diffraction grating are shifted by one-half of the track pitch,
a phase inside cos in the third term in the right hand side is
shifted by .pi.. Accordingly, at this time, I'.sub.0, .+-.1 (x, y),
i.e. interference intensities of the sub-spots, are represented
with reference to u.sub.o, i.e. the spot position of the main spot,
by Equation (9): ##EQU8##
Moreover, in Equation (7), too, if the sub-spots are shifted by
one-half of the track pitch, the tracking error signal is inverted,
too. Accordingly, TR' (.alpha., .beta.), i.e. a tracking error
signal of the sub-spots at the time when there exists the movement
of the lens, is represented by Equation (10):
Consequently, by subtracting the tracking error signal of the
sub-spots from the tracking error signal of the main-spot, a signal
expressed by Equation (11) is obtained: ##EQU9##
Accordingly, when an off-track of the main-spot is equal to 0,
namely, u.sub.o =0, the off-set is represented by the following
Equation (12): ##EQU10##
Consequently, if the width of the groove is not one-half of the
track pitch, the off-set remains. This is caused by the fact that,
as seen from the second term in the right hand side in the upper
stage of Equation (11), when the main-spot is on-track, the
interference intensity thereof differs from interference
intensities of the sub-spots. Thus, by anticipating this intensity
variation and setting in advance a gain G.sub.2 shown in FIG. 4, it
is possible to cancel the off-set. Also, in the case of an optical
disk such as the DVD-RAM in which the land-groove type optical disk
is employed, the off-set is canceled out automatically without
setting such a gain.
The above-described description has been given concerning the
effect of canceling the off-set in the tracking error signal. The
method employed therefor is that a light beam the interference
phase of which is inverted is detected simultaneously. By the way,
this method also cancels out the disturbance into the focus error
signal when an optical spot crosses the guiding groove, which
appears as a serious problem in the astigmatic focal point shift
detecting method. The principle thereof will be explained below:
First of all, there exist two major causes concerning the
above-mentioned disturbance in association with the crossing over
the track in the astigmatic focal point shift detection. One is
astigmatism exerted upon the optical spot on the disk. The other is
a shift in the four-divided optical detector. Here, the explanation
will be given by employing, as the example, a mixture of the
disturbance caused by the astigmatism. Using W.sub.22, i.e. an
aberration coefficient of astigmatism, and .PHI., i.e. direction
angle of astigmatism, a wave surface having the astigmatism is
represented by Equation (13):
.rho.: Normalized radial coordinate in the pupil
.theta.: Polar angle coordinate in the pupil
W.sub.22 : Aberration coefficient of astigmatism
.phi.: Direction angle of astigmatism
This can be rewritten into the Equation (14), using x, y
coordinates of an effective diameter in the pupil:
W(x, y)=W.sub.22 {(x.sup.2 -y.sup.2)cos 2.phi.+2xy sin 2.phi.}
(14)
X,y : Normalized Cartesian (effective diameter) coordinates in the
pupil
Accordingly, assuming that the wave surface having the astigmatism
is diffracted by the optical disk and the 0th order light and the
.+-.1st order diffracted lights thereof are shifted by .+-..delta.
and then are overlapped with each other on the objective lens pupil
surface, a phase difference in the interference added by the
astigmatism can be approximated as a form of Equation (15):
##EQU11##
Then, using this equation, interference intensities between the 0th
order light and the .+-.1st order diffracted lights are represented
by Equation (16): ##EQU12##
Here, as illustrated in FIG. 6, if representative points A, B, C, D
are picked up in a reflected light beam 602, which has astigmatism
and is reflected from the optical disk, interference intensities at
these points are represented by Equations (17) to (20), using
Equation (16): ##EQU13##
Assuming that, basically, these intensities appear without being
varied on the detectors for detecting the focus error, an
disturbance which, as shown in Equation (21), ##EQU14##
is cos waveform-like in shape with reference to the off-track
u.sub.o is mixed into the focus error signal. Here, a focused spot,
in which variations in intensity distribution of a reflected light
beam reflected when the focused spot crosses the guiding groove are
inverted, is generated simultaneously and is added to the focus
error signal. This transaction eventually means that a quantity,
which is obtained by shifting phase .phi. by .pi. and thus by
inverting a sign of the first sin in Equation (21), is added, and
accordingly the disturbance is canceled out.
The difference in reflectance between the guiding groove and the
guiding inter-groove has required the adjustment of the gain with
the use of, for example, the width of the guiding groove. For
instance, the above-described adjustment has become necessary for
the canceling of the tracking error signal off-set in the
differential push-pull method. However, in the canceling of the
disturbance mixed into the focus error signal, the gain adjustment
is unnecessary.
FIG. 7 shows another embodiment for simultaneously detecting the
light beam in which the polarities of intensity distribution
variations of the reflected light beam reflected when the focused
spot on the optical disk crosses the guiding groove are inverted.
In this embodiment, a linear diffraction grating 701, which is
located in parallel to a radial direction of the optical disk, is
employed. Consequently, it turns out that the .+-.1st order
diffracted lights formed by the diffraction grating on the optical
disk are located on the same track as the 0th order light. Also,
consequently, three four-divided optical detection regions 112,
113, 114 constituting an optical detector 702 for detecting the
reflected light beam are located in parallel to a tangential
direction of the optical disk.
Next, using FIG. 8, the description will be given concerning a
detailed structure of the diffraction grating 701 employed in the
present embodiment. The diffraction grating is constituted so that,
as illustrated in FIG. 8, phase of the gratings is inverted with a
period of D.lambda./(2NA.multidot.P) with reference to P, i.e. a
period of the guiding grooves, NA, i.e. numerical aperture of the
objective lens, and D, i.e. an effective light beam diameter for a
diffraction grating-inserted position. This period is equal to an
interval which is determined by shifts of reflected light beams of
.+-.1st order diffracted lights 802, 803 toward a 0th order light
801 of a diffracted light formed by the guiding grooves of the
optical disk. In a diffracted light formed by this kind of
diffraction grating, phase of a wave surface of the diffracted
light is shifted by an amount of .pi. for each period. Remembering
that a diffraction grating is, originally, a hologram, this
phenomenon can be understood easily. The hologram is produced by
performing, on a film such as a photographic dry plate, exposure
and development processings of an interference fringe formed by two
high coherent lights such as laser lights. When the hologram is
irradiated with one of the lights at the time of performing the
exposure processing thereto, the other light is reproduced as a
diffracted light by means of the hologram. Then, as described
above, if the interference fringe is formed by causing an
interference to occur between the light the wave surface of which
is shifted periodically by one-half of the wavelength and the light
the wave surface of which is flat, it is quite natural that the
interference fringe should reflect the phase shift and
discontinuously form a step of one-half of the fringe. Accordingly,
if, conversely, the light the wave surface of which is flat is
launched into such a diffraction grating, it turns out that wave
surface of the diffracted light is shifted periodically by one-half
of the wavelength.
FIG. 9 is a diagram for explaining a manner in which, when a
diffracted light formed by the phase-inverted diffraction grating
is further diffracted by the guiding grooves of the optical disk,
phase shift regions of the resultant diffracted light are
overlapped. The diffracted light by means of the phase-inverted
diffraction grating is further diffracted by the guiding grooves of
the optical disk, and the 0th order light and the .+-.1st order
diffracted lights are overlapped with each other. However, between
the diffracted lights which are adjacent to each other, such as the
0th order light and the .+-.1st order diffracted lights, the phase
shift regions are in contact with each other without
being-overlapped. FIG. 10 summarizes phase differences added by the
phase-inverted diffraction grating at this time between any two of
diffraction orders included in each of the regions indicated by a,
b, c, . . . in FIG. 9. FIG. 10 shows that phase differences between
the lights which have adjacent diffraction orders and make a
contribution to the tracking error signal, such as the 0th order
light and the .+-.1st order diffracted lights, are equal to .pi.
without exception. Moreover, phase differences between the lights
the difference in the diffraction orders of which is equal to 2,
such as the 0th order light and the .+-.2nd order diffracted
lights, are equal to 0. Accordingly, concerning the phase
differences in the interference shown in the Equation (5), without
causing the sub-spots to be off-track by one-half of the track
pitch, it is possible to embody inversion of the interference
intensities which is equivalent thereto. This transaction, even if
storage marks exist asymmetrically on the both sides of the central
spot, brings about no asymmetry in an amount of reflected light of
the sub-spots. Consequently, it becomes possible to further
stabilize the effect of canceling the off-set in the tracking error
signal or the effect of canceling the disturbance into the focus
error signal.
Here, the phase-inverted diffraction grating is not integrated with
the objective lens. Accordingly, it turns out that, when the
objective lens moves following the decentering of the optical disk,
an optical axis of the phase-inverted diffraction grating and that
of the objective lens are relatively shifted with each other. FIG.
11, which indicates the phase shift regions in this case, shows
that the movement of the objective lens, even if it occurs, results
in a mere movement of connected portions between the phase shift
regions, thus bringing about no obstacle to the inversion of the
interference intensities.
FIG. 12 shows still another embodiment for simultaneously detecting
the light beam in which the polarities of intensity distribution
variations of the reflected light beam reflected when the focused
spot on the optical disk crosses the guiding groove are inverted.
Here, instead of the phase-inverted diffraction grating in FIG. 7,
a polarizing phase shifter 1201 is employed. The polarizing phase
shifter 1201 inverts a phase of only a linearly polarized light
component, which is polarized in a specific direction and launched
into the polarizing phase shifter, in regions of a period of
.lambda.D/(2NA.multidot.P). Then, the linearly polarized light
component is split and detected just in front of an optical
detector 702, using a 3-beam Wollaston prism 1202. At this time,
unlike the case of the phase-inverted diffraction grating, there
occurs no sub-spots, and thus there exists only one optical spot on
an optical disk 107. This condition makes it possible to reduce a
loss in the amount of light caused by the sub-spots, thus being
able to constitute an optical head suitable for the writable
optical disks.
FIG. 13 is a diagram for explaining the principle of the polarizing
phase shifter. Here, an example using lithium niobate (LiNbO.sub.3)
is presented. A lithium niobate substrate 1301 has a principal axis
1302 having a refractive index anisotropy in the direction
indicated in FIG. 13. Then, proton exchanged regions 1303 are
formed in the substrate in accordance with grating patterns.
Moreover, in accordance with the grating patterns, dielectric
material layers 1304 are formed. At this time, a phase difference
.phi..sub.o between ordinary rays 1305, 1306 which are each
launched into the grating patterns and therebetween, and a phase
difference .phi..sub.e between extraordinary rays 1307, 1308 which
are each launched into the grating patterns and therebetween, are
represented as the following equations, respectively: ##EQU15##
Here, with the diffraction efficiency taken into consideration,
setting the respective phase differences to be appropriate design
values and solving Equation (22) as simultaneous linear equations
with T.sub.d, i.e. a thickness of the dielectric material layers,
and T.sub.p, i.e. a depth of the proton exchanged regions, as the
unknowns, the solutions are represented by Equation (23):
##EQU16##
, which means that it is possible to design a polarizing grating
which allows desirable phase differences to be created concerning
the ordinary rays and the extraordinary rays independently of each
other. For example, if the desirable result is that: the wavelength
.lambda.=0. 66 .mu.m, the refractive index of the dielectric
material layers n.sub.d =2. 2, .phi..sub.o =0.degree., and
.phi..sub.e =+180.degree., it will do to let T.sub.p =2. 06 .mu.m
and T.sub.d =0.07 .mu.m. Taking the values in this way makes it
possible to selectively shift the phase concerning the proton
exchanged regions by an amount of .pi., thus being able to expect
the same offset canceling effect as that in the above-described
embodiments.
The description will be given below concerning a canceling effect
obtained by a scalar diffraction simulation toward an disturbance
in a focus error detection signal when a focused spot crosses the
guiding groove and a canceling effect obtained by the scalar
diffraction simulation toward an off-set which accompanies the
movement of the objective lens. FIG. 14 shows a focus error signal
at the time when there exist the astigmatism, a spherical
aberration and detector deviations in a complex state in the
ordinary focus error detecting system. The central portion is
swelled, which demonstrates that there occurs a considerably large
disturbance. On the other hand, FIG. 15 shows a calculation result
on the assumption that the phase-inverted diffraction grating is
used. This demonstrates that almost all of the disturbance is
canceled out. FIG. 16 shows a focus error signal obtained by adding
focus error signals of the main-spot and of the sub-spots according
to the differential push-pull method. This demonstrates that almost
all of the disturbance can be canceled out.
FIG. 17 shows a case in which, with the objective lens being moved
and in the ordinary astigmatic focus error detecting method, a
tracking error signal on the light-receiving surface is calculated.
This demonstrates that there occurs a considerably large off-set.
Meanwhile, FIG. 18 shows a case in which the same calculation is
performed using the phase-inverted diffraction grating. This
demonstrates that almost all of the off-set is canceled out.
Furthermore, FIG. 19 shows an embodiment in which the same
calculation is applied in the differential push-pull method. This
demonstrates that the off-set is made extremely small.
FIG. 20 is a diagram for showing an embodiment of an optical system
constitution according to the present invention in which the
reproduction is possible especially in a CD, a CD-R, a DVD-ROM, and
a DVD-RAM. Two semiconductor lasers, i.e. a 650 nm semiconductor
laser 2001 for the DVDs and a 780 nm semiconductor laser 2002 for
the CD and the CD-R, are mounted. In view of spectroscopic
characteristics of reflectance of a CD-R storage film, the 780 nm
semiconductor laser 2002 is absolutely necessary for the
reproduction in the CD-R. The respective lights are launched into
diffraction gratings 2003, 2004, respectively, thus generating the
.+-.1st order diffracted lights. Here, the diffraction grating 2003
for 650 nm wavelength is a diffraction grating described up to now
in the present invention, and the diffraction grating 2004 for 780
nm wavelength is a diffraction grating for forming sub-spots for a
3-beam tracking method usually employed to detect a tracking in the
CD. Then, the light with 650 nm wavelength is reflected at a
dichromatic mirror 2005, passes through a beam splitter 2006, is
reflected at a triangle reflection mirror 2007, and is converged on
the DVD 2009 by a DVD/CD compatible objective lens 2008. Meanwhile,
the light with 780 nm wavelength is reflected at the beam splitter
2006 and at the triangle reflection mirror 2007, and is converged
on the CD or the CD-R disk 2009 by the DVD/CD compatible-objective
lens 2008. Respective reflected lights, by way of the DVD/CD
compatible objective lens 2008 and the triangle reflection mirror
2007, passes through the beam splitter 2006, the dichromatic mirror
2005 and an optical component G, then being converged on an optical
detector 2010.
FIG. 21 is a diagram for explaining the optical component G, a
light-receiving pattern constitution of the optical detector, and
signal calculation methods in a plurality of embodiments obtained
by changing the focal point shift detecting method in the
above-mentioned optical system constitution. When a beam size
detection method is employed as the focal point shift detecting
method, a curvilinear diffraction grating 2101 is employed as the
optical component G. The curvilinear diffraction grating 2101, for
each of a 0th order light and .+-.1st order diffracted lights
generated by the diffraction grating 2003 or 2004, outputs optical
spots to be situated somewhat before a focal point on the optical
detector surface and optical spots to be situated somewhat behind
the focal point thereon as .+-.1st order diffracted lights
generated by the curvilinear diffraction grating 2101. At this
time, diffraction efficiency of the curvilinear diffraction grating
2101 is made large enough. This prevents the 0th order light from
being generated, thus making it possible to decrease the number of
the detection regions. In this way, out of the six optical spots in
total, from a set of the .+-.1st order diffracted lights generated
by the curvilinear diffraction grating 2101, a focal point shift
error signal according to the beam size detection method is
obtained. One of the 0th order lights generated by the diffraction
gratings 2003 and 2004 is received by a four-divided optical
detector, thereby being able to obtain a DPD signal (differential
phase detection) employed in the DVD-ROM. Also, it is possible to
obtain a push-pull signal in which the off-set is canceled out from
one of the 0th order light and the .+-.1st order diffracted lights
generated by the diffraction grating 2003. It is possible to detect
the 3-beam tracking error signal for the CD from a difference in
the amount of light between the .+-.1st order diffracted lights
generated by the diffraction grating 2004. Also, it is possible to
obtain a reproduced RF signal from a total amount of light of the
0th order lights generated by the diffraction gratings 2003 and
2004.
When a double knife edge method is employed as the focal point
shift detecting method, a light dividing prism 2102 is employed as
the optical component G. The light dividing prism 2102 divides each
of the diffracted lights, which are generated by the diffraction
grating 2003 or 2004, into four lights on the optical detector
surface. From the four lights of one of the diffracted lights, a
focal point shift error signal according to the double knife edge
method is obtained. The signals, such as the tracking error signal
according to the push-pull method, the DPD signal, the tracking
error signal according to the 3-beam method, and the RF signal, can
be obtained, as shown in FIG. 21, in almost the same way as in the
beam size detection method.
In these focal point shift detecting methods, selection of the
direction of the divided lines in the optical detector makes it
possible to comparatively suppress, in the deviations and the
aberrations, too, the occurrence of the disturbance which
accompanies the crossing over the guiding groove. Accordingly, in
the present embodiment, the constitution of adding the light the
variations in intensity distribution of which are inverted is not
presented in particular. However, depending on the constitution of
the optical system, it may become necessary from the other
requirements to provide a constitution of the divided lines in
which the disturbance occurs easily. In that case, focus error
signals of lights the variations in intensity distribution of which
are inverted are added to each other, thereby allowing the
disturbance to be reduced in the focus error detecting methods
other than the astigmatic focal point shift detecting method, too.
Consequently, the present invention also makes possible an optical
system constitution which, conventionally, could not be employed
from the viewpoint of the disturbance which accompanies the
crossing over the guiding groove. This characteristic allows a
flexibility in the design to be increased.
When the astigmatic focal point shift detecting method is employed
as the focal point shift detecting method, the optical component G
is unnecessary. The reason is that an astigmatism which occurs when
the lights pass through the dichromatic mirror can be substituted
for the astigmatism for the astigmatic focal point shift detection.
This is based on a principle that, when a focused light is launched
into a parallel flat plate, an astigmatism occurs. Here, in the
focus error detection, as described up to now from the viewpoint of
the disturbance, the focus error signals of the 0th order light and
the .+-.1st order diffracted lights are added to each other.
Conventionally, when the astigmatism is introduced using the
parallel flat plate, the parallel flat plate was inserted in such a
manner as to form an angle of 45 degrees toward the tracks so that
the disturbance which accompanies the crossing over the guiding
groove does not occur easily. This kind of restriction, however,
becomes unnecessary because of the canceling of the disturbance
based on the present invention. Accordingly, in some cases, the
present invention can be effective in making the optical head
compact in the whole size. Also, concerning the tracking error
signal according to the push-pull method, the tracking error signal
the distribution of which is inverted is similarly subtracted. The
other transactions are performed in much the same way as in the
cases in which the other focus error detecting methods are
employed.
According to the present invention, by adding an inexpensive
component such as a diffraction grating to a fixed optical system
without mounting it on the objective lens actuator, it is possible
to fundamentally eliminate the disturbance which occurs in the
focus error signal in association with the decentering of an
optical disk when an optical spot crosses a track on the surface of
the storage film. At the same time, it is possible to fundamentally
cancel the off-set which occurs in the tracking error signal in
association with the movement of the lens.
* * * * *